The large scale synthesis, characterization, and processing of nanostructured materials are part of an emerging and rapidly expanding field of nanotechnology.
Nano and micron sized heat sensitive materials can be manufactured in large quantities by dissolving precursor (in-feed large particles) in a suitable solvent and spray-drying them in a carefully controlled environment. However, particle temperature during drying should not exceed the critical temperature for specific materials above which particles may degrade, burn or explode. Therefore, keeping a relatively low drying temperature is very crucial. However, the low temperature requires a longer residence time for suitable drying to take place and hence unusually long dryer length (or height). Furthermore, the drying gas, the local temperature and residence time have a decisive role in the resultant particle morphology. Depending on heat and mass transfer conditions, dried particle shapes will vary. For example, super heated steam drying can produce more porous materials as compared to hot gas because of the associated high vaporization rate. Industrial drying encompasses a host of drying methods including drum drying, pneumatic dryers, direct and indirect heated dryers, steam dryers, microwave dryers and pulse heat dryers.
Spray drying is a unique drying process since it involves both particle formation and drying. The characteristics of the resultant powder can be controlled, and powder properties can be maintained constant throughout a continuous operation. With the designs of spray dryers available, it is possible to select a dryer layout to produce either fine- or coarse-particle powders, agglomerates or granulates.
Spray drying involves the atomization of feed into a spray, and contact between spray and drying medium resulting in moisture evaporation. The drying of the spray continues until the desired moisture content in the dried particles is achieved, and the product is recovered from the air.
In order to produce “tailored” particle size and morphology, the prior art uses spray drying in horizontal or vertical dryers. Typically, hot drying gas moves upwards while the precursor spray travels down. The hot gas is often swirled.
There are several problems with prior art spray drying that persist. For instance, a need exists for shorter length dryers for low temperature drying in methods where the in-feed precursor solution is atomized and sprayed into the drying chamber. Typically, the drying gas flows either co-current or counter-current. In co-current situations, the spray and drying gas mix downstream inside the chamber. In the prior process technology the dryer length is excessive for low drying-gas temperatures, often exceeding 30-50 meters depending on the dryer gas temperature and solid loading. Therefore, a technology is needed for effective drying that allows the dryer length to remain reasonably short (1-5 meters) for low temperature and high solid loading processes.
Additionally, the mixing efficiency of the spray and drying gas is an important factor in heat transfer and drying rate. An efficient mixing of the spray and gas is required to promote a high heat transfer rate, as well as uniform drying.
Thus, there is a challenge for low temperature drying, such as drying at temperatures less than 100° C. Because of a lack of efficient mixing of spray and drying gases and low drying rates, the required dryer length is presently required to be very long relative to desired usage of space. This is a great manufacturing concern for modern nano and micron particle production of heat sensitive materials such as pharmaceuticals, propellant ingredients, explosives, and high-energy solids.
Another challenge presented by low temperature drying is control of residence time of the spray. Spray momentum may flow downward counter to the upward drying gas flow. The residence time of the spray and thus the particles is a complex function of upward drying gas flow, swirl, and spray momentum.
Finally, there is presently a lack of process technology for low temperature production of nanomaterials using ultra high humidity drying (UHHD), and there are no dryer process technology guidelines for drying heat sensitive materials using UHHD.